Most modern optical scanners and bar code readers ("symbology readers") employ optical systems to direct reflected light from the indicia being read onto an image sensor. Today, multi-element charge coupled device (CCD) arrays may be used as the image sensor, and, with such an array, a bar code symbol can be read while maintaining the reader in a stationary position, or by moving the indicia with respect to the reader, such as in an over-the-belt reader.
Often, because of the demands of an application, a portable point and shoot reader is desirable. However, accurate, portable symbology readers are relatively new and very expensive.
The evolution of the hand-held video camera, or "camcorder", has made available relatively inexpensive (around $100/unit) hand-held two-dimensional array CCD sensor technology. However, as is described below, the CCDs supplied in camcorders do not have adequate resolution to read the more complex symbology in the short exposure times needed for bar code readings, and therefore have not been utilized as symbology readers. This problem is largely due to the way the CCD in the camcorder works and the extremely short time frame in which a symbology needs to be read.
Typically, a spherical lens is used to focus rays reflected from an object onto the CCD. The CCD separates the image to a defined number of parts, called "pixels", which can be thought of as small dots on the CCD which extend in horizontal columns and vertical rows across the CCD. The number of pixels along each direction is referred to as the "sensor resolution" along that direction.
Because spherical lenses compress images of items equally in all directions, the representation of the object on the CCD is a scaled representation of the object. Thus, each pixel represents part of the object of a certain size, and conversely each inch of the object occupies a certain number of pixels in the image, depending on the sensor resolution, the compression of the spherical lens and the distance of the object from the lens. This number of pixels ("dots") per inch of the object is referred to as the "object resolution". The CCD may thus be said to project from the object through the optical system (the spherical lens) a certain object resolution defined by the sensor resolution, the lens characteristics, and the distance of the object from the camera lens. The maximum object size in a given meridian at a particular objection resolution is then determined by multiplying the number of pixels (on the CCD) in that meridian by the object resolution in that meridian (assuming the spherical lens fills the entire CCD).
Typically, a camcorder produces an interlaced video signal having two separate fields. Each field consists of alternating two row scanning, with the rows of pixels in each field aligned in the horizontal direction. Therefore, when viewing the CCD in terms of sensor resolution, each field can be considered to be represented by every other horizontal row of pixels. The fields typically have an exposure time of 1/60 second, which effectively provides a complete new set of pixels for both fields every 1/30 second (the standard IA video frame rate).
Tests have shown that for a point and shoot application, shutter speeds on the order of 1/250 second are required to provide the necessary image freezing for reliable decoding performance in a symbology reader. Because this time frame is far less than the time it takes a camcorder CCD to replace the sets of pixels in both fields, only one field of video signal information can be captured when the shutter is opened. This effectively halves the vertical sensor resolution of the CCD, because an entire field of pixels, making up every other horizontal row, is not used.
Camcorders CCD's have moderate sensor resolution, typically having 768 pixels in the horizontal direction and 494 pixels in the vertical direction. Thus, in accordance with the description above, the sensor resolution for a typical camcorder CCD at a shutter speed necessary for image freezing is 768 pixels in the horizontal direction by 247 pixels in the vertical direction (494/2=247). As can be understood from the description below, the limitation on vertical sensor resolution can limit the horizontal object size to an insufficient amount for reading most bar codes. In addition, when two-dimensional symbols are to be read, it is preferred that vertical object resolution be equal to the horizontal. Without a drastic physical aspect ratio, the above-described discrepancy between the vertical and horizontal resolutions of the sensor makes the object resolution in the two meridians far from the same.
Many bar codes require about 100 dots per inch (dpi) of object resolution in both the vertical and horizontal fields for consistently successful decoding. As described above, the vertical resolution of the sensor is often the limiting factor on both horizontal and vertical maximum object size at a required object resolution. Thus, the horizontal object resolution can be determined assuming 100 dpi vertical object resolution. With the vertical object resolution fixed at this value, the horizontal and vertical object size and horizontal object resolution provided by the typical camcorder CCD at a predetermined length from the camera can be calculated as follows (assuming industry standard of a CCD sensor with a physical aspect ratio of horizontal to vertical of 4/3):
Vertical object size=247 pixels/100 dpi=2.47 inches PA1 Horizontal object size=2.47 inches.times.4/3=3.29 inches PA1 Horizontal object resolution=768 pixels/3.29 in.=233 dpi
The 3.29 inches of horizontal object size is generally unacceptable for industry standards, even if a user points and shoots the reader accurately and hits the bar code perfectly with each attempt. If a reasonable border is allowed around the label for human point and shoot tolerance, the horizontal object size would need to be expanded even more. In addition, the horizontal object resolution of 233 dpi set forth above is excessive and not required. Preferably, for a two-dimensional symbology reading, the horizontal object resolution would be equal to the vertical.
One way to increase the horizontal object size is to use a progressive scanning sensor which doubles the vertical resolution and therefore doubles the horizontal and vertical object size. These cameras are very expensive and currently are only available in prototype form. There is a need for an inexpensive system which can adapt a typical camcorder CCD to accommodate the needed horizontal object size and correct for the discrepancies between horizontal and vertical sensor resolution.